Primeval Demon

Get any world to pure black tendency, so that the Primeval Demon will show up, and be sure to be in Soul Form. It's best to do this in Stonefang (2-1), because the Primeval Demon at 2-1 is located close to the Archstone.

Attack it, with a weak weapon, until it has just enough health left for a one-hit kill.

When you're being summoned, you'll see the usual words flash on the screen. When "Invading as a black phantom" pops up for the second time, kill the Primeval Demon, then mash X to pick up the item it drops. (You will obtain a colorless soul)

You need to die while invading, but have the player you invade kill you. Any other death while invading will result in the loss of a soul level.

You can also use the White Eye Stone to return the moment you are summoned. (This will result in the loss of a soul level.)

When you return to your world, the Primeval Demon will have spawned with full health! Repeat this as much as you would like.

Should you by mistake kill the Primeval Demon when trying to wear its health down, just move on to the next one. You can also try holding the PS button and turning off your system; however this may damage your save data. but only if you quit during an autosave so be sure not to pick up the stone because that will trigger an autosave.

Note: When you finish killing Primeval Demons, and you leave the world, depending on how many Primeval Demons you have killed, your world tendency will be at Pure White. This is because the kills counts toward the World Tendency at the instant of death, while the death itself (the event that trigger the primeval to not spawn again) will only count after the complete HDD saving… and you stopped it by invading

Make sure you are standing near the item, and "Pick up item" (or "Examine") is displayed on screen.

As soon as you use the stone, the message "Searching for another world to invade" will flash three times. After this the message "Invading" will flash three times. Then the message "Invading another world as a black phantom" will flash three times. When the message "Invading another world as a black phantom" flashes for the second or third time, pick up the item but DO NOT CLOSE THE NEW ITEM MESSAGE WINDOW

The idea is to pick up the item as late as possible, so there is no time to save before the loading screen appears.

Get yourself killed by the host. Using the White Eye Stone or suicide will both decrease one soul level from your character. Of course if you want to de-level, then suicide is a good option here.

After you return to your own world, the Colorless Demon's Soul will still be present underneath the tree, but if you check your inventory it will also be present there. Use the Black Eye Stone again and repeat the same process until you have the desired number of Colorless Demon's Soul in your inventory.